The purpose of this study is to examine how environmental and phylogenetic factors interact to affect the social organization of Burchell's and Grevy's zebra. Typically, the social organization of equids has been classified into two types. In one, animals live in permanent groups composed of female, their offspring and a dominant stallion (Burchell's zebra, horses), whereas in the other there are no permanent associations among individuals and males remain solitary, establishing large mating territories (Grevy's zebra, wild asses). Recent studies, however, suggest that species specific social patterns are more apparent than real, and than an equid's social system can change as environmental and demographic factors change. I propose that 1) solitary-territorial systems arise in arid areas because low quality vegetation distributed in small, widely scattered patches prevents the formation of permanent membership female associations by intensifying competition. And without females to defend, males instead defend large areas containing the resources that females require; and 2) harem systems occur in lush areas because the more evenly distributed, higher quality vegetation allows females to associate in permanent groups which males can defend. Underlying the hypothesis is the assumption that intrinsic species differences do not constrain social flexibility. This will be evaluated by comparing aspects of the feeding, ranging, agonistic, mating, and parental behavior in short, the social organization--of each zebra species in its area of allopatry (where only it exists) and in the environmentally different area of sympatry (where both species exist). Intra-specific differences between population living in the center and on the periphery of each species' range will reveal the ways in which environmental factors influence social organization, and will provide the baseline against which to make the inter-specific comparisons in the area of range overlap. The degree to which the social systems converge in this area will reveal the extent to which species specific morphologies and physiologies contain the environmental control of social behavior.